Age hardenable chromium-nickel stainless steel



Patented Aug. 7, 1945 UNITED STATESN PATENT oFFlcE AGE HABDENABLE CHROMIUM-Mcm STAINLESS STEEL Ernest H. Wynne and Raymond smith,

Pittsburgh, Pa.

Application October 8, 1941, Serial No. 414,194

Claims.

' 20% nickel, and the described carbide former.

The amount of the carbide former used is such as to assure its combining with at least maior amounts of the carbon, for in steel of the type under discussion vthe carbide former cannot be used greatly in excess of the amount required to combine with the carbon, since this would cause the steel to lose many of its noteworthy qualities.

The prior art teaches that the physical properties of chromium-nickel stainless steel that does not contain such a carbide former may be adjusted in a practical manner only by the strainhardening method. That is to say, the hardness and strength of the steel may be increased by cold-drawing or cold-rolling, for instance. Unfortunately, this method is not applicable in many instances, such as in the case of castings and fabricated articles. Furthermore, should the steel be strain-hardened while it is in a form permitting the necessary incidental deformation required by this method, and the steel then fabricated into structural parts requiring welding, such as airplane parts, for example, the 'welding operation will anneal the steel at the welded junctions so that, at these points where most hardness and strength is usually needed, the steel will have only the hardness and strength of its annealed structure.

In the case of the type with which this invention is concerned, the prior art teaches that its physical properties may also be adjusted in a practical manner by the age-hardening method, providing it is iirst strain-hardened. Since strain-hardening is necessary, this practice introduces the same limitations as were just described. In addition, the aging progresses so that a maximum in hardness and strength is eventually obtained by this method, after which the hardness and strength rapidly decrease, the steel then being overaged. Therefore, fabrication of this steel, requiring welding, introduces the same dimculties already discussed in this connection, it being necessary to strain-harden the steel while it is in a form permitting the necessary deformation, and to then age while the strain-hardened structure persists. Obviously, subsequent welding results in the welded areas having only the hardness and strength of the annealed structure of the steel, with surrounding zones that are overaged.

Another approach to solving the general problem. of providing a stainless steel of high hardness and strength, is to produce a predominantly nickel steel containing much higher amounts of the described type of carbide former than are consistent with producing a.'workable material, it `having been found that a steel having these compositional characteristics can be aged to greater hardness and strength. However, in time of war, nickel is an element of great strategic importance, so its use in large amounts is impractical. In any event, the large amounts of the carbide former which must be used., in carrying outthis procedure, are deleterious to the quality of the steel.

With the foregoing in mind, the object of the present inventors is to adjust the physical properties -of chromium-nickel stainless steel, of the type with which this invention is concerned, by making this type amenable to aging in a controlled manner, without necessitating its being rst strain-hardened and without necessitating the use of the carbide former in amounts that can introduce trouble,

Heretoiore, it has been recognized that for a chromium-nickel stainless steel to be eifectively amenable to aging, it must contain an element capable of precipitating in the propermanner, but there has been no adequate explanation as to why stainless steels of the stabilized type, that is to say, those containing stronger-than-chromium carbide formers, are not, generally, amenable to aging, regardless of whether they are rst strain-hardened.

The present invention is vpredicated upon the discovery that the aging ability or chromiumnickel stainless steel. containing the required carbide former, depends upon its microscopic structure. More specifically, it has been found that when its structure is either austenite, or well crystallized ferrite, it is not amenable to aging in a practical manner without rst being strainv hardened, but that a highly eiective precipitation occurs, through aging, in the stress-laden form of alpha-ferrite, which results from the transformation of austenite to ferrite at comparatively low temperatures, which, so far as is now known, should not exceed 600 F. and, better, lower temperatures. Assumably, itsformation involves the martensite mechanism.

Experimental work has indicated that the extent of the aging characteristics of chromiumnickel stainless steel, containing the necessary carbide former, is directly proportioned to the amount of stress-laden, low-temperature alphaferrite included by the steel. It follows that. through adjustment of the structure of the steel so as to procure adjusted mounts of alphaferrite which transforms at low temperatures from austenite, followed by aging, overaging or selected heat-treatments, the physical properties of steel of this type may be very nicely adjusted in an accurately predeterminable manner. Since the amount of the proper kind of ferrite included may be adjusted by relatively slight compositional adjustments which do not prevent the steel from being classified outside of the type under discussion, or which impart any objectionable characteristics, it is possible, in the light of this new knowledge, to produce a steel which inherently transforms at low temperatures. either in part or practically entirely, to stress-laden alpha-ferrite, whereupon the steel is amenable to aging regardless of whether it is first strainhardened. In other words, such a steel can be aged to greater hardness and strength whenin its cast, hot-Worked or annealed condition.

With the foregoing in mind, the present invention is characterized by the step, during the making of the Asteel of the type with which it is concerned, of. proportioning the components of the steel that are ferrite formers to those that are austenite formers, to provide the steel with a structure which, to a predetermined extent, transforms to alpha-ferrite upon cooling to a low enough temperature to assure its being stressladen alpha-ferrite, this being followed by the steps of cooling the steel to effect this transformation and aging. This aging may be done by reheating the steel to temperatures substantially above normal atmospheric temperatures and below the temperature causing retransformation of the actively ageable ferrite to austenite, and holding Vthe steel within this range of reheating temperatures for varying time periods as required to adjust the physical properties of the steel to those desired, the correct amount of low-temperature alpha-ferrite produced, being chosen to attain this end.

Since the carbide former which must be included is also a ferrite former, and since it has been explained that the use of large amounts of the carbide former are deleterious to the quality of the steel, it is to be understood that the proportioning required to produce the stress-laden ferrite is not to be construed as being carried out merely by loading the steel up with the carbide former, while maintaining the other components in such proportions as to produce mainly an austenitic steel were it not for this loading. It has already been noted that the Vprior art has approached the problem herein involved by resorting to the use of such high amounts of carbide former as to produce trouble. Therefore, it is to be understood that the ferrite forming components that are proportioned to those that are austenite formers. are components other than the carbide former, while remembering that the latter does play a minor part in producing ferrite and so must be considered in the proportioning calculations. However, it is never to be used in excessive amounts to convert an otherwise austenitic steel into a ferritic steel, for reasons already explained.

Prior to cooling, the structure of the steel is, of course, mainly austenite. Depending upon the proportioning of the ferrite-forming components and those that are austenite formers, more or less of this austenite will transform to ferrite as the steel closely approaches or attains atmospheric temperature under ordinary cooling conditions, such as air-cooling. The low-temperature alpha-ferrite that transforms will, at this time, contain, inV addition to the possible existence of some delta-ferrite. a precipitated phase, which gives the structure an appearance similar to that of low-carbon martensite, when 'it is viewed under the microscope. Aging produces a precipitate of still another phase, resulting from the use of the carbide former and which is believed trbe an intermetallic compound, which, as aging progresses, acquires an optimum, or critical, particle size giving the steel maximum aged hardness and strength. after which it agglomerates so that the hardness and strength materially lessen, this being in agreement with the recognized aging phenomena.

The steel is completely amenable to age-hardening after its low-temperature austenite-toferrite transformation, without first requiring strain-hardening. At the same time, strainhardening does not interfere in any way with this susceptibility to age-hardening, it increasing this susceptibility in the'event the steel is produced with a structure that is less than practically entirely low-temperature alpha-ferrite, since austenite included by the structure of the steel with such ferrite, will more or less transform to stress-laden alpha-ferrite, the amount that transforms depending on the amount of strainhardening, and the particular composition involved.

Due to the age-hardening characteristics of the steel, it is for the first time possible to increase the hardness and strength of castings, these being completely amenable to aging when the present invention is followed. Likewise. prior to aging, the steel may be rolled or drawn to shapes which are then fabricated into structural parts requiring welding, such as are required in airplane constructions, and the hardness and strength of the entire stmcture then materially increased by aging the same. 'Ihis permits aging of the welded areas, as well as all other parts, and for the first time permits the. attainment of great hardness and strength in such instances throughout the entire system that is subject to stress. Since strain-hardening does not interfere with the aging of the steel, it may be subjected to cold-drawing and cold-rolling when these operations are necessary, such as to obtain a better surface or in connection with cold-forming operations. The corrosion resistance of the new steel is not impaired as compared to the corrosion resistance common to prior art steels of its general type. r

More specifically speaking, the invention contemplates the making of a steel of the described type within the following percentage composition range:

.50 to 3.0 Balance iron.

a,se1,41e

These percentage composition ranges may be narrowed to obtain a critically more vigorous response to asini l! fo11ows:

Carbon .05 to .08 Chromium 15 to 18 Nickel 4 to 'i Titanium.. .2 plus 5 to 12 times the carbon content Aluminum .0e to 1.0 Menganeee.abmit-- .so for 1% Ni or.- Lilli-3.5 for 5% Ni or loll-6.0 for 3% Ni In all events, the components that are ferrite this element used. 'and the or columbium, which are'also the strong carbide iormers,

'must -be proportioned respecting the carbon,-

nickel and manganese, these representing the austeniteeformlng components, to produce a steel having a structure thattransforms at low temof the structure alpha-ferriteat sub--l -trnsformation temperatures, although as little as permits enioymentfof the aging -phenomena to'a small degree without ,the necessity for the strain-hardening formerly required. The age-hardening obtainable. is proportional to the amount of low-temperature alpha-.ferrite developed. l

In connection with the above it has already been explained that the.-.proportioning must be calculated in view of the ferrite forming effect of the carbide former, but that the carbide formera: namely. the. chromium, aluminum if.

The titanium or columbium, whichever is used, is of course necessary. In either instance these elements should be adjusted to from about five to twelve times the carbon content, with a few tenths of a per cent. extra for satisfying any' nitrogen which might b'e present in the steel and with which such elements combine.

A. steel produced with the composition disclosed herein may have a structure including, as previously mentioned, as much as 90% austenite with the remainder alpha-ferrite in the form of low-carbon martensite. although maximum response is obtained when the subtransformation structure is l100% martensite. Such a steel need only be cooled in the air in the normal manner for it to be responsive to aging, and the degree of aging obtained can be adjusted by controlling the amount of low-temperature alpha-ferrite through proper compositional adjustment, and through control of the heat treatment used to age. As previously mentioned, it does not require strain-hardening; although this will not prevent it from responding to aging. The development of a very small amount of low-temperature alpha-ferrite permits small cold reductions, from 5 to,20%, to have about the same effect as the heavy reductions formerly required.

As a specific example of the invention, steel of the type with which this invention is concerned had its composition adjusted so that it contained, in per cent., .06. carbon, 16.8 chromium, 7.25 nickel, .50 titanium. and .30 aluminum, with the main balance iron, the other elements being of a composition normal to this type of steel, which is classifiable as 18-8, titanium-stabilized, stainless. When hot-rolled to 18 gauge, various manipulations of this steel gave the following results:

- raid Percent Omnium R2?" 0.2% U.'r.s. erin sono P. s. i. P. s. i. A asset-relied es 100,000 iaaooo 1 2s e mni-wem---- s as aan .s 27 n .smnoplmeon-sb'xfa'i -IL se uaooo mono 11 former must be to amounts not harmful It has been found that the steel of this invento the quality of the steel.

Although the use of aluminum is optional, it is preferably used to the extent of a few ounces perton-of-steel to assure complete deoxidation so as to prevent loss of the relatively expensive titanium or columbium. Incidentally, zirconium is also known to be operative when used in proper amounts, and it is believed that the other stronger-than-ehromium carbide formers used in-the type of steel here involved will also be operative since vthe aging phenomena is attributed to the precipitation of the carbides of such elements. However, for various reasons, titaniu-m or eolumblum are the preferred .carbide formera. v

When aluminum is used. the chromium con'- tent is advantageously .lowered by about 1% for each .2% of residual aluminum in the steel, aluminum being a'ferrite former and thus permitting these reductions of this other ferrite former, chromium. It is preferred, however, to use aluminum to the` extent of net over .20%. although higher percentages, such as might be used to combat' particle.corrosion problems. do not prevent the steel from beinginherently aging.

. time periods of from about five minutes up to one hundred hours. This breadth is possible because alpha-ferrite that forms close to room temperature, will not retransform to austenite until reheated to much higher temperatures, i. e., temperatures above the age-hardening range. In each instance, for each temperature, a critical time period will be found, which produces the maximum hardness and strength. Furthermore,

it will be found that there is a peak temperature producing maximum hardening and strengthening in the least time. Gveragins may be effected, for the purpose oi softening and improving the ductility. by heating at the same temperature for a longer time period than was found to be critical, or, more expeditiously, by heating above what was found to be the peak temperature for the steel under treatment. Generally speaking.

overaging may be effected in any manner known to result in an s gglomeration of the precipitate causing the phenomena.

It is also possible to soften and weaken the steel by heat treatment, in a variety of ways.

Thus, the steel may be heated slightly above the temperature where its` alpha-ferrite transforms to austenite, and then cooled, or it may be heated to a lower temperature, which is at the same time well above the top aging temperature, and then cooled, both of these procedures resulting in a steel that is thereafter relatively nonaging. Maximum softness is obtained by heating well above the heating transformation temperature of the steel, followed by a retarded cooling down to a temperature below that where aging occurs, usually 200 or 300 F., after which the steel may be cooled at a faster rate to room temperature. This last treatment produces a steel which remains non-magnetic in its austenitic form at room temperature for from several minutes to several days, or more, depending on analysis. With time, at room temperature, the austenite transforms to alpha-ferrite, whereupon the steel becomes inheibntly aging. Coldworking will also cause this transformation. It follows that the steel may have its physical properties adjusted in practically any desired manner.

It has been already mentioned that the present invention is practically applicable to the fabrication of highly stressed structural parts, such as airplane parts. 'This is attributed to the fact that it permits a new welding method in connection with chromium-nickel stainless steel of the stabllizedtype, the described step of adjusting the composition of the steel, permitting aging after the forming and welding of the steel, whereby to produce a structure of great hardness and strength. In addition to this, a fact that further ilts the steel for use in connection with airplane parts and similar highly loaded structures, is that the steel has a rather long range of loading over which the deformation is linear with the applied' stress. In this connection, the steel may be lightly cold-worked to the extent of a reduction of from to 20%, and then aged, whereupon it is not uncommon to obtain proportional limit values in excess of 140,000 p. s. i. When compared with ordinary high-tensile 18-8 of comparable `strength, this improvement is noteworthy, such steel having a proportional limit value not greater than from 50,000 to '70,000 p. s. i.

In addition to the above. the new steel obtains its great strength by heat treatment as contrasted to cold-working, and, therefore, has the same strength in all directions as contrasted to the directional strength characteristics of a coldworked product. For instance, ordinary hightensile 18-8 frequently only has about one-half the yield strength in compression that it has in tension, depending on rolling direction, and hence cannot be used safely in highly stressed beam parts, for example. The new steel may be safely used under such circumstances.

'I'he accompanying drawing graphically illustrates some of the phases ofthis invention that have been under discussion, as follows:

Figure 1 shows the eil'ects obtained when a chromium-nickel stainless steel of the stabilized type has its composition adjusted, as disclosed herein. to obtain varying amounts ofv low-temperature alpha-ferrite;

Figure 2 shows the same as Figure 1 in connection with a relatively small amount of strainhardening: and

Figure 3 shows the response to aging for thirty minutes at various temperatures, inA the case of this steel.

More particularly, in Figures 1 and 2, the ordinates represent the hardness in strength, as i1- lustrated, specifically, by the ultimate tensile strength, while the abscissas represent the per cent. of stress-laden alpha-ferrite included by the steel; the amount in the case of Figure 1 being due solely to compositional adjustments, while in Figure 2 it is due to this cause and, in addition, to the effects of strain-hardening obtained by a 17% cold reduction. It is to be understood that in both instances the steel is a chromium-nickel steel of the stabilized type having chromium and nickel contents within ranges preserving the desirable characteristic of such steel, and containing, in these instances, either titanium or columbium in adequate amounts, but which are, however, not excessive'insofar as the stabilized type is concerned.

Observation o f Figure 1 shows that the aging response of the steel, when plotted against its low-temperature alpha-ferrite content, is substantially linear, the broken line representing the steel in its annealed condition, while the solid line represents the steel'in properly aged condition. As previously remarked, the response of the steel to aging is directly proportional to the per cent. of low-temperature alpha-ferrite included by the steel. The Vper cent. austenite, or face-centered lattice structure, and ferrite or body-centered lattice structure, can be calculated by X-raydiffraction and the microscope, as is well known in the art.

In the case of Figure 2, the broken line is the annealed steels response solely to the 17% coldworking, While the solid line is the annealed and cold-worked steel which has, in addition, been properly aged. Since strain-hardening is addi- 40 tionally involved, the low-temperature-ferrite-tostrength relationship is no longer linear throughout. The term annealing, as above used, refers to the treatment given 18-8; namely, cooling from 2000 F.

' Going to Figure 3, this graph shows the response of ordinary annealed 18-8 titanium stabilized steel, represented by the broken line, and that of a steel having its composition adjusted so it contains, in per cent., 0.07 carbon, 17 chromium,7 nickel, .85 titanium, and .15 aluminum, this being represented by the solid line and being annealed before aging in an identical manner; i. e., cooled from 2000 F. In both instances, the aging consisted in holding the steel for thirty minutes at the various temperatures shown by the abscissas, the ordinates showing the hardness as indicated by Brinell hardness. y

It is to be noted that the ordinary stabilized 18-8, as annealed by cooling from about 2000 F., does not respond to aging at all, whereas the steel having its composition adjusted so that it contains an adequate amount of low-temperature alpha-ferrite responds quite noticeably within the temperature range of from 300 to 1100" F. The maximum response is obtained around 900 F. to 950 F., the latter being the temperature of aging used in developing the graphs in Figures 1 and 2. However, it is to be understood that the use of longer or yshorter temperature-holding periods will result in a different curve from that illustrated by Figure 3. Furthermore, the substantially flat portion of the curve above about 1200 F. may be slightly higher or lower than shown, dependingu'pon the particular composition and heat treatment.

We claim:

1. A method of adjusting the physical properties of carbon-containing chromium-nickel stainless steel of the type containingA at least one of the stronger-than-chromium carbide formers such as titanium, columbium, zirconium, etc., said method comprising the step, during the making of said steel, of proportioning its components other than saidcarbide formers that are ferrite formers to those that are austenite formers to provide said steel with a structure that at least partly transforms at relatively low temperatures to alpha-ferrite upon cooling, with any remainder austenite followed by the steps of cooling said steel to produce said alpha-ferrite and aging.

2. A method of adjusting the physical properties of carbon-containing chromium-nickel stainless steel of the type containing at least one of the stronger-than-chromium carbide formers such as titanium, columbium, zirconium, etc., said method comprising the step, during the making of said steel, of proportioning its components other than said carbide formers -that are ferrite formers to those that are austenite formers to provide said steel with a structure that at.

least partly transforms at relatively low temperatures to alpha-ferrite upon cooling, with any remainder austenite followed by the steps of cooling said steel to produce said alpha-ferrite and aging, said aging bein'g done by holding said steel to temperatures substantially above normal atmospheric temperatures and below the temperature causing retransformation of said ferrite to austenite, for varying time 'periods as required to adjust the physical properties of said steel to those desired.

3. A chromium-nickel stainless steel of the type containing carbon and at least one of the stronger-than-chromium carbide formers such as titanium, columbium, zirconium, etc., said steel being characterized by having its components other than said carbide formers that are ferrite formers proportioned to those that are austenite formers to provide said steel with a structure that includes a predetermined amount of low-temperature ferrite, with any remainder austenite and by being aged to develop a uniform precipitate other than cementite and delta-iron throughout said ferrite, which has a particle size adjusted to provide said steel with desired plnrsical properties.

4. A chromium-nickel stainless steel of the type containing carbon and at least one of the stronger-than-chromium carbideV formers such as titanium, columbium, zirconium, etc., said steel being characterized by having its components other than said carbide formers that are ferrite formers proportioned to those that are austenite formers to provide said steel with a structure that includes a predetermined amount of lowtemperature ferrite, with any remainder austenite and by being aged to develop a uniform precipitate other than cementite and delta-iron throughout said ferrite, which has a critical particle size providing said steel with a maximum aged strength.

5. An aging stainless steel containing from .03 to .15% carbon, from 12 to 20% chromium, from 2 to 10% nickel, from :25 to 10% manganese, from .40 to 2% titanium, and the balance mainly iron with a structure that at least partly transforms to stress-laden ferrite upon cooling with any remainder austenite.

6. An aging stainless steel containing from .03 to .15% carbon, from 12 to 20% chromium, from 2 to 10% nickel, from .25 to 10% manganese, from .50 to 3% columbium, and the balance mainly iron with a structure that at least partly transforms to stress-laden ferrite upon cooling with any remainder austenite.

7. Age-strengthened chromium-nickel stainless steel having substantially the same strength in all directions and free from the directional strength characteristics of a cold-worked product, with a structure that is partly low-temperature ferrite and partly austenite.

8. A method of adjusting the physical properties of chromium-nickel stainless steel of the type containing carbon and at least one of the stronger-than-chromium carbide formers such as titanium, columbium, zirconium, etc., said method comprising the step, during the making of said steel, of proportioning its components other than said carbide formers that are ferrite formers to those that are austenite formers to provide said steel with a structure that at least partly transforms at relatively low temperatures to alphaferrite upon cooling, with any remainder austenite and retransforms to wholly austenite only at relatively high temperatures, followed by the steps of cooling said steel below the first-named temperature and reheating it thereabove to less than the second-named temperature to effect accelerated aging.

9. An aging stainless steel containing from .03 to .15% carbon, from 12 to 20% chromium, from 2 to 10% nickel, from .25 to 10% manganese, up to 1% aluminum, from .40 to 3% of alloy of the class consisting of titanium'and columbium with the titanium ranging from .40 to 2% or the columbium ranging from .50 to 3%, and with' the balance mainly iron, said steel having a structure that at least partly transforms to stress-laden ferrite upon cooling with any remainder austenite.

10. An aging stainless steel containing from .05 to .08% carbon, from 15 to 18% chromium, from 4 to '1% nickel, .2% plus 5 to 12 times the carbon content of titanium, from .03 to 1% aluminum, manganese ranging from about .50% for 7% nickel, from 1 to 2.5% for 5% nickel and from 3 to 6% for 3% nickel and with the balance mainly iron, said steel having a `structure that at least partly transforms to stress-laden ferrite upon cooling with any remainder austenite.

ERNEST H. WYCHE. RAYMOND SMITH. 

